Large storage tanks are often cylindrical and have a circular floating roof. The roof floats on the surface of the liquid, thereby decreasing the vapor space inside of the tank. A floating roof may be required for reasons of safety or for pollution reduction. The floating roof has a seal to seal it to the wall of the tank that helps to prevent the escape of the contained liquid or vapors from that liquid.
There are two broad types of storage tanks that utilize floating roofs: tanks having an exposed floating roof and tanks having a fixed roof covering the floating roof. An advantage of the covered tank is that it protects the floating roof from undesirable effects from the external environment, such as rain or birds. A disadvantage of the covered tank is that volatile, explosive, corrosive, or toxic gasses or liquids can accumulate between the floating roof and the fixed roof.
The floating roof is a large dynamic structure having a diameter of up to 100 meters and weighing several tons. This dynamic structure is subjected to changing forces from environmental conditions (temperature, wind, etc.) that affect the tank, the lid, or the contained liquid; convective forces within the liquid; or forces that occur when liquid is added or removed, including friction from the seal. The roof will flex and possibly tilt in response to these changes, which may result in the loss of the contained liquid or vapor into the space between the floating roof and the fixed roof. In extreme cases, the roof may tilt enough to cause it to sink into the tank.
The industry is therefore quite interested in monitoring systems that can be used to improve safety, reduce environmental concerns, or avoid major problems such as seal failure or a sunken roof.
There are existing patents that address the application of electronic monitoring or control to storage tank systems. For example, U.S. Pat. No. 4,596,266 (Kinghorn, et al., 1986) describes an electronic safety valve and system for controlling the roof drain on a hydrocarbon storage tanks for the purpose of allowing water drainage while preventing the escape of the contained liquid. U.S. Pat. No. 4,176,553 (Wood, 1979) describes a system for measuring the level of a liquid in a storage tank having a predetermined reference level. Although these patents are applied to storage tanks, they are essentially unrelated to the invention described herein. There is no existing art that addresses the roof monitoring system described herein.
U.S. Pat. No. 6,700,503 (Masar, et al. 2004) describes a means for wireless remote monitoring and graphically displaying the liquid level inside of a tank. The invention described herein does not incorporate a graphical display.
WIPO PCT filing 94/26627 (Christensen, 1994) describes a system for estimating the inclination of a storage tank roof by using float-based liquid level sensors and reed switches whereas the invention described herein uses solid-state micro electro-mechanical devices to directly measure inclination.
There is a considerable body of literature, standards, and patents that describe wireless sensor networks. A representative book that describes the current art is Protocols and Architectures for Wireless Sensor Networks by Karl Holger and Andreas Willig (Wiley, 2005).
U.S. Pat. No. 7,339,957 (Hitt, 2008) describes how transmission time slots are allocated to transmitting nodes in a system of wireless environmental sensors and actuators for the purpose of controlling irrigation systems. Although the invention described herein utilizes environmental sensors, it does use actuators nor does it rely upon a slotted communications protocol.
U.S. Pat. No. 7,386,352 (Davis, et al., 2008) describes a modular sensor network node architecture where the node architecture has each sensor coupled to its own small microprocessor so that it can be “plugged” into a sensor node containing a master microprocessor. The invention described herein is not modular and requires only a single microprocessor.
U.S. Pat. No. 7,468,661 (Petite et al. 2008) describes a system and method for monitoring and controlling remote devices. This patent describes variety of application areas but does not address storage tank monitoring. In its Detailed Description, this patent “describes the data structure of messages sent and received using the invention”. Throughout its claims, the communications system requires a retransmission device (e.g., “one or more retransmitting receivers”) or a “computer program that formats and stores select information for retrieval on demand”. The invention described herein does not require any of these components.